The present invention relates generally to oxygen-binding heme proteins, and in particular to such proteins incorporating one or more hemoglobin tetramers incorporating at least one functional, circularly-permuted globin.
As further background, blood transfusions allow trauma patients means to replenish blood loss, surgery patients to enter longer procedures with less risk, and rescue workers to bring a blood supply to accident victims. Although a transfusable blood supply provides many benefits, available blood is limited by human donations. In addition, the limited shelf-life of whole blood, disease transfer, and mismatched blood typing are problems yet to be fully addressed.
For example, the occurrence of an HIV-contaminated blood supply in the 1980's heightened awareness for a need to circumvent the problems associated with a donated blood supply. Even today, the United States Department of Health and Human Services has created a blood safety panel to examine many issues relative to donated blood, including HIV and hepatitis.
Transfused blood, containing plasma, white blood cells (leukocytes), platelets and red blood cells (erythrocytes), is generally used to carry oxygen from the lungs to the rest of the body's cells. A number of oxygen carrying solutions are being studied as alternatives to blood transfusions. In this regard, an effective blood substitute must satisfy three basic requirements. First, it must transport oxygen from lungs to tissues. Second, it must remain functional in vivo long enough to be effective; and third, it must not elicit harmful side effects. Blood substitutes studied to date include perfluorocarbons (Kaufman, R. J. (1991) in Biotechnology of Blood (J. Goldstein, e., Ed.) pp. 127-162, Butterworth-Heinemann, Boston), chemically modified hemoglobin from outdated human blood (Winslow, R. M. (1992) Hemoglobin-based red cell substitutes, Johns Hopkins University Press, Baltimore), and recombinant hemoglobins produced in microbial and mammalian hosts (Shen, T. -J., Ho, N. T., Simplaceanu, V., Zoiu, M., Green, B. N., Tam, M. F., & Ho, C. (1963), PNAS USA 90, 8108-8112; Rao, M. J., Schneider, K., Chait, B. T., Chao, T. L., Keller, H. Anderson, S., Manjula, B. N., Kumar, R., & Acharya, A. S. (1994) ACBSIB 22, 695-700).
On the subject of hemoglobin, each hemoglobin molecule is a tetramer of four smaller polypeptide subunits known as globins. A heme group, which is an iron-protoporphyrin complex, is associated with each polypeptide subunit, and is responsible for the reversible binding of a single molecule of oxygen. Normal adult hemoglobin is made up of two different kinds of polypeptide globins. A first globin, known as alpha globin, contains 141 amino acid residues. The second, known as beta globin, contains 146 amino acid residues. In normal adult hemoglobin, two of each kind of globin are arranged in the form of a truncated tetrahedron which has the overall shape of an ellipsoid.
The overall hemoglobin molecule is a 64,400 kDa protein. X-ray crystal structures show the size of HbAo to be about 64 .ANG..times.55 .ANG..times.50 .ANG. (Fermi, G., Perutz, M. F., Shaanan, B. and Fourme, B. (1984) Journal of Molecular Biology 175, 159). The heme prosthetic group of each alpha subunit is non-covalently bound to the subunits by Lys E10, His CD3, Val E11, and Phe CD1. In beta chains, His CD3 is replaced by Ser CD3 The heme contains an Fe++ bound by the proximal histidine. A distal histidine hovers over the iron but does not coordinate; however, this histidine could sterically and/or electronically hinder the binding of CO, which has a higher affinity for heme than O.sub.2, as well as hydrogen bond to iron in the deoxystate. The irons in the hemes can oxidize to the Fe+++ state, creating a nonfunctional hemoglobin (Bunn, H. F. a. F., B. G. (1986) in Hemoglobin-Molecular, Genetic, and Clinical Aspects (Dyson, J., Ed.) pp. 13-19, W. B. Saunders Company, Philadelphia).
Ligands that bind hemoglobin include CO, NO, CN--, and the most physiologically relevant ligand, O.sub.2. Oxygen binding occurs in a sygmoidal pattern, demonstrating the cooperativity of multiple ligand binding. It has been shown that hemoglobin can exist in at least two states, T and R. The T state is associated with the deoxygenated state of hemoglobin, while the R state is associated with ligand bound hemoglobin. A number of models have been offered to describe the shift from T to R when ligand is bound. Two primary models describe the change in states as either a concerted change from T to R or a sequential change of subunits from T to R as ligand is bound. The concerted model proposed by Monod, Wynman, and Changeux describes cooperativity resulting from the entire tetramer converting from T to R (Monod, J., Wyman, J., and Changeux, J. -P. (1965) Journal of Molecular Biology 12, 88-118). The induced fit mode describes cooperativity as the result of an R state, ligand bound subunit inducing a neighboring T state subunit to alter to the R conformation (Koshland, D. E., Nemethy, G. and Filmer, D. (1966) Biochemistry 5, 365-385). Recently, Ackers and co-workers have proposed a symmetry model for T to R transition which provides evidence for an intermediate state in T to R transition (Ackers, G. K., Doyle, M. L., Myers, D., and Daugherty, M. A. (1992) Science 255, 54-63). The eight intermediate ligation states have been studied using metal-substituted hemes that are unable to bind ligand. The evidence demonstrates the steepest free energy change occurs when a subsequent ligand binds the alternate alpha/beta dimer.
Ligand affinity is also dependent on a number of allosteric effectors. The effectors that lower oxygen affinity include protons (Bohr effect), 2,3 diphosphoglycerate, and chloride ions. The physiological relevance of the effectors is to enhance oxygen delivery to metabolically active cells that produce CO.sub.2.
Modification of human hemoglobin has been widely investigated as a means to provide a blood substitute and for other uses. Hemoglobin is a well-characterized protein, and can be altered to meet the basic requirements for an effective and safe blood substitute. Chemically modified, and more recently, recombinant forms of hemoglobin, are currently being tested in various stages of clinical trials.
Some problems arise from overproduction of recombinant hemoglobin in prokaryotes and eukaryotes. In humans, methionine aminopeptidase recognizes small, hydrogphobic residues as a signal to cleave (Hernan, R. A., Hui, H. L., Andracki, M. E., Noble, R. W., Sligar, S. G., Walder, J. A., & Walder, R. Y. (1992), Biochemistry 31, 8619-8628). Therefore, the first amino acid in postranslationally modified human hemoglobin is a valine. However, during the expression of human hemoglobin in E. coli, the initial methionine is not cleaved. Further, E. coli methionine peptidase recognizes small polar side chains, and expression in E. coli essentially adds a methionine to the primary sequence of both alpha and beta chains. This issue has been dealt with in two ways. A yeast expression system has been utilized in which the initial methionine is cleaved (Wagenbach, M., O'Roueke, K., Vitez, L., Wieczorek, A., Hoffman, S., Durfee, S., Tedesco, J., & Stetler, G. (1991) Bio-Technology 9, 57-61). In prokaryotic production, the replacement of the first amino acid - valine - with a methionine was used in both alpha and beta chains (recombinant hemoglobin des-val) to produce a protein functionally similar to HbAo (Hernan, R. A., Hui, H. L., Andracki, M. E., Noble, R. W., Sligar, S. G., Walder, J. A., & Walder, R. Y. (1992), Biochemistry 31, 8619-8628).
It has been reported that these overproduced hemoglobins are misassembled in the yeast and E. coli (Hernan, R. A., & Sligar, S. G. (1995) JBC, 270, 26257-26264). The misassembled tetramer initially binds ligand similarly to wild type hemoglobin, but over time drifts to different tetramer substrates that bind ligand at different rates. The drift appears to be time and temperature dependent, and protein stored at -70.degree. C. still encounters a drift problem. Wild type hemoglobin stored at -70.degree. C. has not demonstrated a similar effect.
Studies have shown that hemoglobin blood substitutes offer a number of difficulties as well as benefits. Hemoglobin is a powerful tool for oxygen delivery, but its use removes a tightly regulated protein from its native environment. One major problem for hemoglobin based blood substitutes occurs when oxygen in the heme iron dissociates as superoxide ion, leaving hemoglobin oxidized in the ferric "met" state. This autoxidation leaves hemoglobin in a state where it cannot bind ligand. Moreover, the Fe+++ state is an intermediate in the pathway to the highly reactive Fe.sup.4 + ferryl state, heme loss, and can cause peroxidation of lipids (Giulivi, C., and Davies, K. J. A. (1994), Methods of Enzymology 231, 490-496; Yamamoto, Y., and La Mar, G. N. (1986) Biochemistry 25, 5288-5297; Repka, T., and Hebbel, R. P. (1991) Blood 78, 2753-2758). Superoxide off-rates appear to govern the measured autoxidation rate.
Another key problem associated with hemoglobin-based blood substitutes is hemoglobin's affinity for nitric oxide (NO), which is higher than its affinity for CO or O.sub.2. NO is a vasodilator and can be carried by hemoglobin as a heme ligand or on a cysteine as a nitrosothiol (Bonaventura (1996) Nature 380, 221-226). Results in clinical trials demonstrate that patients treated with a hemoglobin-based blood substitute often encounter higher blood pressure (Blantz, R. C., Evan, A. P., and Gabbai, F. B. (1995) in Blood Substitutes: Physiological Basis of Efficacy (Winslow, R. M., Vandegriff, K. D., and Intaglietta, M., Ed.) pp. 132-142, Birkhauser, Boston). Another problem with hemoglobin is that the molecule is small enough to extravasate into the endothelial lining and bind NO. Patients treated with L-arginine, an intermediate in the NO synthesis pathway, or nitroglycerine, a vasodilator, have normal blood pressures while being administered hemoglobin solutions (see Blantz, R. C., Evan, A. P., and Gabbai, F. B. (1995) in Blood Substitutes: Physiological Basis of Efficacy, supra.
Perhaps the most significant drawback of hemoglobin blood substitutes is the rapid filtration of hemoglobin molecules by the kidney. At concentrations used in patients, hemoglobin dissociates into alpha/beta dimers small enough for renal filtration. This not only significantly decreases the lifetime of the blood substitute (half life of less than an hour), but it also deleteriously effects renal tubules and can cause renal toxicity (see Blantz, R. C., Evan, A. P., and Gabbai, F. B. (1995) in Blood Substitutes: Physiological Basis of Efficacy, supra).
One important step in eliminating renal toxicity is the cross-linking of alpha/beta dimers. Current efforts include chemical cross-linking of two alphas or two betas with a covalent attachment to lysine residues (Vandegriff, K. D., & Le Telier, Y. C. (1994) Artificial-Cells-Blood-Substitutes-and-Immobilization-Biotechnology 22, 443-455). In addition, hemoglobins have been randomly polymerized using glyceraldehyde (Vandegriff, K. D., & Le Telier, Y. C. (1994) Artificial-Cells-Blood-Substitutes-and-Immobilization-Biotechnology 22, 443-455). However, utilization of a chemical reaction significantly lowers the yield of functional protein.
Researchers have produced a genetically cross-linked hemoglobin molecule with a half life of almost two hours (Looker, D., Abbott-Brown, D., Cozart, P., Durfee, S., Hoffman, S. Mathews, A., Miller-Roehrich, J., Shoemaker, S., Trimble, S., Fermi, G., Komiyama, N. H., Nagai, K., & Stetler, G. L. (1992) Nature 356, 258-260). X-ray crystallography has shown the C-terminus of one alpha chain to be only 2 to 6 .ANG. away from the N-terminus of the second alpha chain (Shaanan, B., (1983) Journal of Molecular Biology 171, 31-59), and trypsin catalyzed reverse hydrolysis has demonstrated that an additional amino acid attached to the C-terminus does not alter oxygen binding properties. These results, coupled with the knowledge that the C-terminal arg141 can form a salt bridge with the alternate alpha chain's vall, demonstrated the feasibility of genetically cross-linking the two alpha chains. The di-alpha chain expressed by these workers in E. coli consisted of an alpha des-val, a glycine linker, and a native alpha chain sequence. The construct was co-expressed with a des-val version of a naturally occurring low-oxygen affinity beta mutant (beta Presbyterian, R108K), and the entire construct was dubbed rHb1.1.
Despite these extensive efforts to develop a hemoglobin-based blood substitute, needs still exist for substitutes with increased crosslinking and higher molecular weight, which provide increased molecular stability and plasma half-life, and a decreased risk of renal toxicity. Such substitutes will desirably be readily expressed in host cells in high yield and have advantageous oxygen-binding capacity. The present invention addresses these needs.